1. Field of the Invention
The present invention relates to a toner for electrostatically charged image development used for developing an electrostatic latent image formed by electrophotography or electrostatic recording, a method for manufacturing the toner, an image forming method using the toner for electrostatically charged image development, and an image forming apparatus using the image forming method.
2. Description of the Related Art
A method for visualizing image information via an electrostatic image by, for example, electrophotography has been used in various fields in recent years. An image is visualized in the electrophotographic method through the steps of forming an electrostatic latent image on a photoreceptor by charging and exposing the photoreceptor, developing the electrostatic latent image with a developer containing a toner, and transferring and fusing the image.
A two-component developer including a toner and a carrier, and a one-component developer using only a magnetic or nonmagnetic toner are known as the developer used in the method. The toner contained in the developer is usually produced by a kneading-pulverization method in which a thermoplastic resin is melted and kneaded with a pigment, a charge control agent and a releasing agent such as wax, and the mixture is cooled, pulverized and classified. Inorganic or organic fine particles may be added onto the surface of the toner particles, if necessary, in order to improve fluidity and the cleaning property of the toner. While this method can provide a quite excellent toner, it involves some problems as set forth below.
The shape and surface structure of a toner produced by the conventional kneading-pulverization method becomes irregular. Although subtle changes are possible depending on the pulverizability of the materials used and pulverization conditions, it is difficult to purposely control the shape and surface structure of the toner. Moreover, the range of selection of the materials is restricted in the kneading-pulverization method. Specifically, a dispersion of a resin and a colorant must be pulverizable with an economically practical production apparatus.
However, finer powders may be undesirably generated by a mechanical shear force applied to a toner in a developing apparatus or the shape of the toner may be changed, when the resin colorant is made to be more fragile to satisfy the above requirements. These effects may cause accelerated deterioration of charging characteristics of the developer due to adhesion of the fine powder on the surface of the carrier in the two-component developer, or scattering of the toner due to expanded particle size distribution and deterioration of image quality by a decrease in developability due to the change in the toner shape in the one-component developer.
When a large amount of the releasing agent such as wax is used in producing a toner, the releasing agent is often excessively exposed on the surface of the toner depending on a combination between a thermoplastic binder resin and the releasing agent. Such exposure of the wax on the surface of the toner is evident particularly in a combination between a resin that includes a polymer component to enhance elasticity and that is a little difficult to pulverize and a fragile wax such as polyethylene or polypropylene.
Exposure of the wax component is advantageous for removing the toner from a fusing roll in the fusing process and for washing non-transferred toners from the surface of a photoreceptor. However, the wax component exposed on the surface of the toner may be readily transferred onto a member of an image forming apparatus when a mechanical force is applied onto the toner during use. Accordingly, a development roll, the photoreceptor and the carrier are liable to be contaminated, and thus reliability may decrease.
Furthermore, as described above, the shape of the toner is irregular. Therefore, sufficient fluidity is difficult to obtain even if a fluidizing aid is applied to the toner. Consequently, fine particles added to the surface of the toner move to concave portions on the surface of the toner due to a mechanical shear force during the use of the toner, and fluidity of the toner decreases over time or the fluidizing aid is embedded in the toner to impair developability, transferring property and cleaning property. Image quality may further decrease when a toner retrieved by cleaning is returned to a development unit and used again. However, when the amount of the fluidizing aid added to the toner is increased to prevent such adverse effects, black spots are generated on a photoreceptor and the fluidizing aid particles are scattered.
A method for producing a toner by an emulsification-polymerization aggregation method has been proposed in recent years as a means for enabling the shape and surface structure of the toner to be purposely controlled in order to solve the problems caused by irregularity of the toner shape (see Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439). In this method, the toner is produced by mixing a dispersion liquid of resin fine particles prepared by emulsification-polymerization and a dispersion liquid in which a colorant is dispersed in a solvent, forming aggregate particles having a diameter corresponding to the diameter of toner particles to be formed, and heating the aggregate particles to fuse and integrate the aggregate particles.
The shape of the toner produced by this method may be controlled to a certain extent, which improves chargability and durability of the toner. However, since the inner structure of the toner becomes almost uniform, removability of a recording medium on which an image is formed from a fusing roll in a fusing process, stabilization of transparency of an image on an OHP sheet, and different charging among colors remain as problems to be solved when an image is formed using the toner.
As described hereinbefore, exposure of the releasing agent on the surface of the toner should be suppressed in order to permit the toner to maintain stable performance under various mechanical stresses, and the surface hardness of the toner should be enhanced to suppress deterioration of fusability of the toner in the image forming method by an electrophotographic process using the toner. In other words, the toner's own mechanical strength should be enhanced, and chargability and fixability of the toner should be compatible at a high level.
Since a demand for high image quality has grown in recent years, the diameter of the toner particles has been remarkably decreased to obtain images of high definition in forming color images. However, the problems of contamination of the carrier and the photoreceptor, and scattering of the toner become evident due to toner particles having an extremely small diameter, when the toner diameter is simply reduced while the particle diameter distribution remains the same. As a result, it is difficult to simultaneously realize high image quality and high reliability. Therefore, the particle diameter distribution should be narrowed while the diameter of the toner particles is reduced.
In digital full-color copiers and printers, colors of a color image manuscript are separated with B (blue), R (red) and G (green) filters to obtain blue image information, red image information and green image information, and latent images which are composed of dots having a diameter in the range of 20 to 70 μm and which correspond to the original manuscript are formed in accordance the image information and developed by taking advantage of a subtractive color mixing using Y (yellow), M (magenta), C (cyan) and Bk (black) developers. However, a larger amount of the developer must be transferred in such a process than in conventional monochromatic printing. Since the development process is required to be able to accurately develop latent images composed of dots having a small diameter, uniform chargability, persistence of the charge, toner strength and sharpness of the particle size distribution are becoming more and more important in recent years.
Low temperature fixability is also required for the toner considering high speed and energy-saving operation of the apparatus. A toner suitable for low temperature fusing may be produced by taking advantage of a method for producing the toner using an emulsification-polymerization aggregation process suitable for producing toner particles having a small diameter.
On the other hand, it is necessary that large amounts of color toners are sufficiently mixed in a full-color printer, and improved color reproducibility and transparency of an image on an OHP sheet are essential for full-color printing.
Meanwhile, a polyolefin wax is usually used as the releasing agent component of the toner in order to prevent low temperature offsetting in the fusing process. A trace of a silicone oil is uniformly applied onto a fusing roll in order to prevent high temperature offsetting in the fusing process. Accordingly, since the silicone oil adheres onto the surface of a recording medium on which an image is formed, the surface gives an unpleasant sticky feeling when the recording medium is handled.
A toner for oilless fusing which contains a large quantity of the releasing agent component has been proposed in order to solve the problems as described above (see JP-A No. 5-061239). Removability of the toner from a fusing member may be improved to a certain extent by adding a large amount of the releasing agent. However, it is difficult to obtain stable removability, since a problem of compatibility between a binder resin and the releasing agent is caused and the releasing agent unevenly bleeds on the surface of the toner. In addition, since the means for controlling aggregation force between the toner and the binder resin depends on the weight average molecular weight Mw and glass transition temperature Tg of the binder resin, it is difficult to directly control spinnability and coagulability of the toner in the coalescence process. Further, free components in the releasing agent may cause inhibition of charging.
In order to solve the problems in oilless fusing, a method for enhancing rigidity of the binder resin by adding a high molecular weight component to the binder (see JP-A Nos. 4-69666 and 9-258481), and a method for improving rigidity of the binder resin by chemically introducing cross-links to the binder so as to eventually decrease spinnability of the toner at a fusing temperature and improve removability of the toner were proposed (see JP-A Nos. 59-218460 and 59-218459).
Various measures for the releasing agent have been investigated and proposed to solve the problems offusability, and particularly removability, of a toner for oilless fusing, transparency of a full color image on an OHP sheet, and/or inhibition of fluidity of the toner powder caused by the releasing agent.
Particularly, a method for using a releasing agent which has a melting point in an intermediate temperature region and which is amorphous or has low crystallinity such as an ester wax has been proposed to improve removability of a recording medium on which an image is formed from a fusing roll at the time of oilless fusing (see JP-A No. 6-337541). According to this proposal, removability of a recording medium on which an image is formed from a fusing roll at the time of oilless fusing is improved by lowering the melt viscosity of the releasing agent, and reduction of transparency of a full color image on an OHP sheet is prevented by using a low crystallinity releasing agent.
However, since the releasing agent component often plasticizes the binder resin component and consequently the Theological property of the binder resin at the time of fusing deteriorates, removability of the toner from a fusing roll of an oilless fusing unit is impaired.
In order to solve the above problems (to suppress a decrease in removability due to plasticization), it becomes inevitable to introduce cross-linking structures into the binder resin, to increase the molecular weight and the glass transition temperature Tg of the binder resin, and/or to add a large quantity of the releasing agent to the toner.
However, countermeasures as described above may often cause a decrease in luster of the image and also decrease transparency of the image on the OHP sheet. In addition, the production cost increases since a large quantity of the releasing agent is required for producing the toner. Furthermore, a releasing agent layer is formed on the image due to a large amount of the releasing agent remaining on the fixed image. The releasing agent layer formed on the image may deteriorate the image quality due to generation of contact traces on the releasing agent layer by contact between a roll for ejecting a recording material out of the image forming apparatus and the image formed on the recording material. Such a phenomenon becomes more evident on an image having higher luster.
Accordingly, there is a need for a toner for electrostatically charged image development which is excellent in removability in oilless fusing, maintains good luster, has excellent fixability such as luster of the surface of a fixed image and transparency of an image on an OHP sheet, can provide images of high definition by suppressing contact traces of ejecting rolls generated in ejecting the fixed image, and renders the image less dependent on an operation environment. Moreover, there is a need for a method for producing such a toner, an image forming method using the toner for electrostatically charged image development, and image forming apparatus using the image forming method.